Method of utilitzation of high dielectric constant (hdc) materials for reducing sar and enhancing snr in mri

ABSTRACT

A method for enhancing the performance of an imaging system that includes providing a magnetic resonance imaging system; providing an object to be imaged, wherein the object has a shape, includes a region of interest therein, and has a first dielectric constant; providing a pad that conforms to the shape of the object and contains a material having a second dielectric constant that is higher than the first dielectric constant; surrounding the pad and the object being imaged with at least one radio frequency coil; using the at least one coil to generate a magnetic field having an intensity of 3 T that extends through the material of the pad and into the region of interest of the object being imaged, and wherein positioning the at least one coil to surround the pad and the object increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object; using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged and to convert the detected magnetic field into detectable electrical signals; and increasing signal to noise ratio of detected electrical signals and decreasing required power input to the at least one coil through the placement of the at least one coil around the pad.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 14/450,957, filed on Aug. 4, 2014, which was a continuation of U.S. patent application Ser. No. 12/958,385, filed on Dec. 1, 2010 (now U.S. Pat. No. 8,798,719), which claimed priority to and the benefit of U.S. Provisional Patent Application No. 61/266,109, filed on Dec. 2, 2009, the disclosures of which are hereby incorporated by reference in their entirety for all purposes.

STATEMENT REGARDING FEDERALLY-FUNDED RESEARCH

This research was supported by grant number R01 AG02771 from the NIH. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

This invention relates generally to the radiofrequency (RF) electromagnetic field, denoted as B1, used for magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) instruments. Particularly, the invention relates to the use of materials with high dielectric constant (HDC) or permittivity with low electrical conductivity for RF field generation in the MRI or NMR instruments.

RF coils are used to produce RF magnetic field (B1) for excitation and detection of the magnetization signal from an object such a part of a human body during MRI and MRS data collections. The RF field can be generated by a coil or a set of coils. The RF field is transmitted into the sample or human body to excite the nuclear spins. Subsequently, the RF signals from the nuclear spins are received by the same or a different set of RF coils. The present invention improves the efficiency of RF coils for both transmission and reception of the B1 field. The efficiency of an RF coil includes B1 field uniformity and intensity in a region of interest (ROI) produced by unit input current. In general, a stronger B1 field generated by an RF coil with unit input current translates to better reception sensitivity and lower RF heating of the tissue samples that may be hazardous to a human body when it reaches to a certain level. The reception sensitivity of an RF coil is experimentally determined by measured image signal to noise ratio (SNR) from an ROI obtained with standardized imaging protocols, and the heating effect by an RF field can be assessed numerically with calculation of Specific Absorption Rate (SAR) distribution in a human body (1).

The SNR of an MR image is critically important for the quality of the image as MRI is an intrinsically low SNR instrument. Significant amounts of effort and resources have been invested to gain higher image SNR in a given image data acquisition time. In particular, this includes the use of higher static magnetic field strength from current 1 to 1.5 Tesla to 3 to 7 Tesla, which is extremely expensive. Unfortunately, an increase in static magnetic field strength leads to a higher frequency of RF field, which, in turn, dramatically increases RF heating effect of the tissue (1) and creates RF field inhomogeneity artifacts (2-5). These two problems pose serious challenges for high field MRI development in human imaging. The present invention introduces a novel method of use of HDC materials that increases B1 field intensity inside the sample and reduces SAR in the sample during image acquisition.

The recent experimental data in high field (3-8 Tesla) human imaging systems demonstrated that high dielectric constant of a human body plays an important role for RF field behavior in a human body in high field. The electrical properties, geometry, and relative position of the sample in the coil become important factors in determining the B1 field distribution inside the sample (6-11). Consequently, adjustment of B1 field distribution inside the sample or human body and the coupling between the sample and coil can be facilitated with HDC materials. Foo, et al. proposed a method of correcting for the RF inhomogeneity in a human body observed on a 4 Tesla MRI system by “dielectric loading of the coil-to-shield space in an RF resonator (coil and shield assembly)” (10). Based on Foo's theoretical analysis, he proposed to adjust the RF homogeneity by loading the coil-to-shield space with dielectric material of suitable relative permittivity so as to alter the propagation constant of the coil. With theoretical calculations, Foo predicted that a value of between 30 and 40 for the relative permittivity of the dielectric material in the coil-to-shield space would reduce the RF field inhomogeneity from +/−15% to about +/−3% over a central 30-cm-diameter region of a homogeneous 40-cm-diameter body at both 64 MH and 170 MHz corresponding to a 1.5 and 4 Tesla MRI system respectively. However, their experimental results at 4 Tesla showed that “the improved RF field homogeneity would be accompanied by increased RF power requirements and reduced coil sensitivity.” There at least three distinctive differences in Foo's work from the present invention. 1) The dielectric material is inserted in the coil-to-sample space in the present invention, while it was loaded in “the coil-to-shield space” in Foo's work. In fact, the dielectric material is placed in the opposite side of the RF coil of the present invention. As demonstrated in their experimental results, Foo's approach produced totally opposite outcome i.e. an “increased RF power requirements and reduced coil sensitivity”. 2) The theoretical bases are totally different. In the present invention, the HDC material is used to couple the RF field produced by the RF coil with the sample. In Foo's work, the dielectric material was used to manipulate “the axial propagation constant of RF resonator” itself. 3) in Foo's work, the choice of dielectric material was based on the specific design and geometry of the RF coil or resonator. In principle, the present invention requires no knowledge of the RF coil configuration. It is more effective, however, that HDC-pads of the present invention are developed to fit a specific coil design for optimal effect.

To address the same RF field inhomogeneity issue in the head image taken at 4 Tesla, Alsop et al. presented a novel spiral RF volume coil design for high field MRI use (2). Images acquired with his spiral coil design showed a signal drop on the top of the human head. It was attributed as the abrupt change in dielectric property between tissue and air at the top of head since his coil design theoretical analysis was based on a mathematical model of an infinite long cylinder. To mitigate this additional problem associated with the spiral coil design, a dielectric pad at the end of the coil was included during image data collection. In his work, the introduction of a dielectric pad was specifically used only for compensation of the signal drop caused by the use of spiral coil. In the present invention, HDC-ads are placed inside an RF volume coil of any design to improve the efficiency of a given RF coil.

SUMMARY OF THE INVENTION

The following provides a summary of certain exemplary embodiments of the present invention. This summary is not an extensive overview and is not intended to identify key or critical aspects or elements of the present invention or to delineate its scope. However, it is to be understood that the use of indefinite articles in the language used to describe and claim the present invention is not intended in any way to limit the described system. Rather the use of “a” or “an” should be interpreted to mean “at least one” or “one or more”.

In accordance with one aspect of the present invention, a first method for enhancing the performance of an imaging system is provided. This method includes providing an imaging system, wherein the imaging system is a magnetic resonance system; providing an object, wherein the object is imaged by the imaging system, wherein the object being imaged has a shape, wherein the object being imaged includes a region of interest therein, and wherein the object being imaged has a first dielectric constant; providing a pad, wherein the pad conforms to the shape of the object being imaged, wherein the pad contains a material having a second dielectric constant, and wherein the second dielectric constant is higher than the first dielectric constant; surrounding the pad and the object being imaged with at least one radio frequency coil; using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; using the at least one coil to convert the detected magnetic field into detectable electrical signals; and increasing signal to noise ratio of detected electrical signals and decreasing required power input to the at least one coil through the placement of the at least one coil around the pad.

In accordance with another aspect of the present invention, a second method for enhancing the performance of an imaging system is provided. This method includes providing an imaging system, wherein the imaging system is a magnetic resonance system; providing an object, wherein the object is imaged by the imaging system, wherein the object being imaged has a shape, wherein the object being imaged includes a region of interest therein, and wherein the object being imaged has a first dielectric constant; providing a pad in the form of a helmet adapted to receive different volumes of material, wherein the pad conforms to the shape of the object being imaged, wherein the pad contains a material having a second dielectric constant, and wherein the second dielectric constant is higher than the first dielectric constant; surrounding the pad and the object being imaged with at least one radio frequency coil; using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; using the at least one coil to convert the detected magnetic field into detectable electrical signals; and increasing signal to noise ratio of detected electrical signals and decreasing required power input to the at least one coil through the placement of the at least one coil around the pad.

In yet another aspect of this invention, a third method for enhancing the performance of an imaging system is provided. This method includes providing an imaging system, wherein the imaging system is a magnetic resonance system; providing an object, wherein the object is imaged by the imaging system, wherein the object being imaged has a shape, wherein the object being imaged includes a region of interest therein, and wherein the object being imaged has a first dielectric constant; providing a pad, wherein the pad conforms to the shape of the object being imaged, wherein the pad contains a material having a second dielectric constant, and wherein the second dielectric constant is higher than the first dielectric constant; surrounding the pad and the object being imaged with at least one radio frequency coil; using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; using the at least one coil to convert the detected magnetic field into detectable electrical signals; and increasing signal to noise ratio of detected electrical signals by 27% and decreasing required power input to the at least one coil by 50% through the placement of the at least one coil around the pad.

It is desirable to have a strong and uniform B1 distribution in the entire sample being imaged. It is an object of the present invention that materials (liquids, solids or mixtures) with high dielectric constant (HDC) or permittivity are incorporated into RF coils to change the RF field to a desirable distribution inside the sample. Here, the word “high” refers to the value of dielectric constant of a material that is higher than that of the tissue of the sample under investigation at the frequency transmitted and received by the RF coil. The presently preferred embodiments are in forms of a pad or pads filled with HDC materials, referred to as HDC-pad thereafter.

It is another object of this invention that HDC materials are incorporated into RF coils to reduce the transmission power for a given excitation flip-angle in a region of interest (ROI) inside the sample.

It is another object of this invention that HDC materials are incorporated into a set of RF receiving coils to enhance the image SNR of the sample.

For certain applications in the art, it is desirable to have a strong and uniform B1 only in a part of the sample or an ROI in the sample. It is another object that HDC materials are incorporated into the coil or inside the sample to enhance an RF coil performance in terms of an increase in SNR in an ROI near or around HDC materials in the sample and a decrease of input RF power, therefore, SAR in the sample during excitation of magnetization.

It is another object of this invention that HDC materials are used as an RF field trap to reduce radiation loss of high field MRI coils. In high field MRI (3 T, 4 T, 7 T and above), radiation loss becomes more significant as the corresponding RF field frequency increases.

It is another object of this invention that HDC materials are used as contrast enhancement agents.

The principle of this invention is to insert layers or coats of materials with high dielectric constant and very low conductivity in between RF coil or coil conductive elements and sample. The HDC materials stated herein can be solids, such as ceramics, or liquids, such as water, deuterium or water-based gel or suspensions of any dielectric additives to adjust the dielectric constant. The geometry, amount, mechanical and electric properties of the HDC material best suited for the embodiments of the present invention will depend on specific applications, RF coil configuration and the resonance frequency of the RF field. For water-based gel pad, the signal from the pad can be removed by addition of a small amount of Manganese Chloride (MnCl₂) or any other NMR signal relaxing compound or by replacing water with deuterium (D2O).

Additional features and aspects of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the exemplary embodiments. As will be appreciated by the skilled artisan, further embodiments of the invention are possible without departing from the scope and spirit of the invention. Accordingly, the drawings and associated descriptions are to be regarded as illustrative and not restrictive in nature.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a part of the specification, schematically illustrate one or more exemplary embodiments of the invention and, together with the general description given above and detailed description given below, serve to explain the principles of the invention, and wherein:

The foregoing and other novel features and advantages of the invention will become more apparent and more readily appreciated by those skilled in the art after consideration of the following description in conjunction with the associated drawings, of which:

FIG. 1A provides an illustration of a computer model of a bird-cage coil.

FIG. 1B provides an illustration of a bird-cage coil surrounding a human head with an HDC-pad.

FIGS. 2A-2C provide images of elliptical ROI used to evaluate T2-weighted image intensity for SNR measurement and uniformity assessment on mid-axial, sagittal, and coronal planes, wherein FIG. 2A is an image of the mid-axial plane, wherein FIG. 2B is an image of the sagittal plane, and wherein FIG. 2C is an image of the coronal plane.

FIGS. 3A-J provide images illustrating that the addition of an HDC-pad resulted in dark regions in central regions of all brain images due to over-tipping with the RF power level optimized without the HDC-pad. The images shown in FIGS. 3K-3T were acquired using input RF power with the attenuation values listed below. The images shown in FIGS. 3P-3T were obtained with the final power attenuation for the 90° flip angle being 14.5 dB, which was 3-dB more attenuation than without the HDC-pad.

FIGS. 4A-4C are axial (FIG. 4A), sagittal (FIG. 4B), and coronal images (FIG. 4C) through the brain with an HDC-pad, and FIGS. 4D-4E are axial (FIG. 4D), sagittal (FIG. 4E), and coronal images (FIG. 4F) through the brain without an HDC-pad. Measured SNR values are given under each image.

FIG. 5A is an image of a calculated B1+ map at 128 MHz in a first orthogonal plane with an HDC-pad, FIG. 5B is an image of a calculated B1+ map at 128 MHz in a first orthogonal plane without an HDC-pad, and FIG. 5C is the B1+ distribution plotted along the dotted lines in FIGS. 5A and 5B. FIG. 5D is an image of a calculated B1+ map at 128 MHz in a second orthogonal plane with an HDC-pad, FIG. 5E is an image of a calculated B1+ map at 128 MHz in a second orthogonal plane without an HDC-pad, and FIG. 5F is the B1+ distribution plotted along the dotted lines in FIGS. 5D and 5E. FIG. 5G is an image of a calculated B1+ map at 128 MHz in a third orthogonal plane with an HDC-pad, FIG. 5H is an image of a calculated B1+ map at 128 MHz in a third orthogonal plane without an HDC-pad, and FIG. 5I is the B1+ distribution plotted along the dotted lines in FIGS. 5G and 5H. All the B1+ maps are scaled to 1 W input power.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention are now described with reference to the Figures. Reference numerals are used throughout the detailed description to refer to the various elements and structures. Although the following detailed description contains many specifics for the purposes of illustration, a person of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following embodiments of the invention are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.

A preferred embodiment of the present invention will now be described in detail with reference to the Figures. Those skilled in the art will appreciate that the description given herein with respect to those figures is for exemplary purposes only and is not intended in any way to limit the scope of the invention. All questions regarding the scope of the invention may be resolved by referring to the appended claims.

Theoretical Considerations

For conductive dielectric materials such as human brain tissues, the RF field inside the sample is perturbed by conductive current (Jc) and displacement current (Jd) according to Ampere's Law with Maxwell's correction,

∇×B=μJc+μJd=μσE+iμεrεoωE

where B is magnetic flux density, E is electric field, ω is angular frequency, εr is relative electric permittivity (dielectric constant), εo is the electric permittivity in vacuum, σ is electrical conductivity, m is magnetic permeability, and i=is the complex unit which introduces a 90-degree phase difference between conductive current and displacement current (Johnk 1988). For plane waves traveling in a homogeneous medium, the conductive current leads to decay of the RF field in the direction of propagation, while the displacement current with a 90° phase shift acts as a secondary field source facilitating RF wave propagation. In this case, the opposing contributions of the two sources to B1 can be considered using ratio Jc and Jd given by

Jc/Jd=σ/ωεoεr.

In principle, materials with low σ and high εr can enhance the local B1 field strength for an RF field frequency range high enough to induce the displacement current to a much stronger conducting current. This equation describes the relationship within the dielectric materials. Subsequently, the RF wave propagates into the sample with stronger amplitude enhanced by the HDC materials. Thus, in general, placement of HDC-pads near an ROI in MRI should result in enhanced local B1 field strength with concomitant improvement of SNR and reduction of overall SAR.

HDC-Pad Design for Human Head Imaging

FIG. 1B illustrates geometry and configuration related to the RF coil and the human head of the preferred embodiment of present invention. The embodiment of the present invention is an HDC-pad in a form of a helmet, containing approximately 6 liters of distilled water, was conformed around the superior portion of the head, extending to just above the level of the eyes in front, just above the ears on the sides, and just below the end of the skull in the back of the head. The thickness of the HDC-pad is 3 mm.

Computer Modeling

The quantitative evaluation on how placement of an HDC-pad changes the B1 field distribution depends on the detailed geometries and sizes of the coil and sample in this embodiment of the present invention, and thus, must be determined numerically with computer modeling. In the following a computer modeling on an HDC-helmet is used to demonstrate the efficacy of the invention.

A numerical model with finite difference time domain (FDTD) method was used to calculate the RF field distribution in the sample and coil model shown in FIG. 1A at 128 MHz corresponding to a static magnetic field strength of 3 Tesla. All the FDTD calculations were performed with commercially available software (XFDTD; Remcom, Inc, State College, Pa.), and post-processing of the simulation results was performed with home-built programs in MATLAB (The Mathworks, Inc., Natick, Mass.). A 30 mesh with isometric 2 mm resolution was created within a region of 57×50×42 cm3. The calculation was performed with −35 dB convergence to ensure that the steady state was reached. A Liao boundary condition was used for the outer boundary truncation of the grid (12). The coil was modeled after that used in experiment: a copper 12-rung high-pass birdcage coil (32.8 cm i.d. and 25.4 cm length, shield diameter of 40.0 cm). The coil model was driven using 24 current sources spaced evenly on the two end rings, at the locations of capacitors in the actual coil. The human head model used for the FDTD calculation included 16 types of tissue and the corresponding electric properties at 128 MHz were HDC-pad was modeled with a 3 cm thick uniform layer of water (σ=0.0047 S/m2, εr=78) over the head excluding the face as shown in FIG. 1B (13).

Experimental Measurements

Human brain images were acquired on a 3 T whole body system (Bruker, Biospin, Ettlingen, Germany) using a quadrature 12-element high-pass birdcage coil with 26 cm inner diameter and 29 cm length. Axial brain images were acquired with identical imaging parameters with and without an HDC-pad placed around the head and after the coil was tuned and matched, and RF power was calibrated for each condition. The subject remained in the magnet during the process of placing and/or removing the HDC-pad, re-tuning the coil and adjustment of RF power for 90°/180° flip angle. Input power for the flip angle was adjusted manually with and without the HDC-pad while maximizing the total signal on 5 axial slices covering a 2.5 cm slab through the center of the brain. Fast spin-echo (RARE) images with slice thickness=5 mm, matrix=128×128, FA=180°, and FOV=30 cm were acquired on five axial, sagittal, and coronal planes spaced 5 mm apart through the cerebrum. The experiment was repeated four times with two human subjects. All of the subjects provided written informed consent prior to participation, in accord with the requirements of the Institutional Review Board of the Pennsylvania State University College of Medicine.

Signal-to-noise ratio (SNR) was measured using the magnitude images acquired under the above two conditions. The average signal intensity was calculated in an elliptical ROI covering most of the cerebrum in each of the 15 images acquired with and without the HDC-pad. Examples of the elliptical region in each orientation are shown in FIGS. 2A-2C. The noise was measured by calculating the standard deviation in an ROI covering 29 cm2 in a region of no visible signal or artifact across the top of each image.

Results

FIGS. 3A-3T show a set of axial images of human brain after addition of the HDC-pad and retuning of the coil, but before re-optimization of the RF power levels (using power values optimized without the HDC-pad) on the left columns, and images with the HDC-pad using the RF power levels indicated at the bottom of columns of the images. After addition of the pad, the RF power levels that previously generated 90°/180° excitation/refocusing pulses in the brain created much larger flip angles due to enhancement of the B1 field. A 3-dB reduction of power for both the excitation and refocusing pulses was required to maximize the signal and images were acquired shown on the far-right column (FIGS. 3P-3T).

FIGS. 4A-4F show images acquired with and without the HDC-pad on axial, sagittal, and coronal planes. The SNR in the brain measured in the elliptical regions shown in the Figures is listed below each corresponding image. For all 15 planes acquired, the SNR increased by 20 to 40% (in average, 27%) with addition of the HDC-pad while the overall uniformity of image intensities appears to be similar in both cases. The SNR of repeated experiments are within the same range.

FIGS. 5A-5I show the calculated B1+ distributions with and without the HDC-pad in three orthogonal planes arranged as in the experimental results. All the fields correspond to 1 W of power delivered to the coil. The calculated B1+ distributions under the two conditions presented similar characteristics as the in vivo images in FIGS. 4A-4F. The RF field appears to be greater inside the brain regions surrounded by the HDC-pad but lower in the regions in the face and neck outside of the padded region. B1+ outside the upper portion of the head and pad is lower when the HDC-pad is present, and the same input power is maintained. This can be seen in the line plots at the bottom of the figure, which present B1+ along the dotted lines in the maps above in FIGS. 5A-5I. The black and gray lines indicate B1+ for cases with and without the HDC-pad respectively. The RF field around the coil elements and in between the coil element and shield (seen most clearly in the middle line plot) was reduced more than 70%, indicating less current is required in the coil to maintain the same RF power level when the pad is present. In ROIs similar to those used in the in vivo results, the average B1+ in the simulation is seen to increase by 12-20% with HDC-pad while the source current in the coil is reduced. These computer modeling results demonstrated that the HDC-pad improves the efficiency of generation of B1 field by a given RF coil, resulting in an increase in image SNR and decrease of RF power required for a given flip angle.

Addition of an HDC-pad surrounding the head resulted in a reduction of required RF power by approximately 50% and an increase in image SNR by approximately 27% with a transmit/receive volume coil at 3 T. No obvious local bias field induced by HDC-pad in the entire cerebrum was observed in the images in FIGS. 4A-4F. In addition, image uniformity within the cerebrum appeared to be somewhat improved. The standard deviation of the signal intensity distribution in the elliptical regions on all 15 images acquired from approximately the same brain volume decreased by an average of 12% with addition of the HDC-pad. The uniformity in this brain area that is normally interference by the so-called bright center spot due to the RF wave effect was reduced markedly.

Comparing images in FIGS. 4A-4F acquired with and without the HDC-pad showed that the image intensity in the cerebrum relative to the neck and face is much greater with the HDC-pad than without it. It is likely that this is due to the enhancement of the B1 field for a given input power in the regions that were surrounded by the HDC-pad. B1 field distribution of the coil is altered by the strong displacement currents in the region of the pad, resulting in stronger B1 fields in the vicinity of the HDC-pad. These experimental data suggested that the regional enhancement of the B1 fields in the ROI can be extended to the entire cerebrum with proper coverage by the HDC-pad while the B1 field outside the sample is decreased. As a direct result, average SAR levels for the brain imaging will be lowered with the HDC-pad, and because SNR is proportional to the ratio of the B1 field strength to the square root of the corresponding RF power, SNR is increased with addition of the HDC-pad. The numerical modeling results summarized in FIGS. 5A-5I are consistent with the experimental data and clearly demonstrate the enhancement of the B1 field in the brain by the HDC-pad. The B1 field is increased in the entire cerebrum region while outside the brain appears to be reduced by the placement of the HDC-pad when the B1+ maps are normalized to unit input power. Under this condition, B1+ is increased in the cerebrum with reduced input current in the coil. This is apparent in the middle line plot, where the two ends of the line pass through coil current elements. Since the magnetic field in the upper portion of the head is relatively high compared to elsewhere, a greater percentage of the input power is also dissipated in that region. To maintain the same total input power as is the case in FIGS. 5A-5I, the coil requires less current and produces lower B1+ in regions outside the HDC-pad compared to the case when the HDC-pad is not present. Thus, overall, the presence of the HDC-pad improves efficiency of delivery of both B1 field and RF energy to the section of the head surrounded by the HDC-pad.

Those skilled in the art will appreciate that further development of the present invention will lead to an even greater improvement in reducing SAR and improving regional SNR in MRI. The in vivo data presented here at 3 T suggested that HDC-pads around the head or other parts of the anatomy could be used to enhance performance of an RF coil in a variety of cases. An HDC-pad with adjustable volume could be used to enhance RF coil performance while simultaneously providing comfort and reduction of patient motion. This could be particularly beneficial for pediatric patients since most RF coils are designed to accommodate larger adult anatomies. In some cases, there may also be advantages to incorporating dielectric material directly into RF coil constructions. Further developments of the various embodiments of this invention include determination of the locations, dimensions, geometries and permittivity distributions of the material for optimal B1 enhancement.

Water was used in this embodiment as a dielectric medium to demonstrate the desired effect of the present invention as water has relatively high dielectric constant and low conductivity, is readily available, inexpensive, and nontoxic. From a technical point of view, however, water is unlikely to be the most suitable dielectric material for many intended applications in the art because it produces strong signal that saturates the receiver and decreases the dynamic range of the digitizer and its movements and geometry are difficult to control. Deuterium (D2O) and high dielectric constant material such as barium titanites slurry suspension in the deuterium can be used to replace the water. It is known to the art also that certain ceramic materials have a dielectric constant as high as a few thousand, which can be used for the embodiments of the present invention.

Those skilled in the art will appreciate that strategic placement of HDC-pads around the head within a given RF coil at 3 T can result in reduced RF transmission power and improved image SNR throughout the cerebrum, and that with further exploration and development, use of HDC-pads may provide a relatively simple and low-cost method for improving quality and safety of MRI in a variety of applications.

Those skilled in the art will also appreciate that numerous other modifications to the preferred embodiment and other embodiments of the present invention are possible within the scope of the invention. These include further developments in optimization of the size, shape, thickness and volume of HDC-pads for specific applications to given body parts or organs for MRI systems with various static magnetic field strengths; in formulation and processing of high dielectric materials used for the HDC-pads; in selection of the values of dielectric constant (permittivity) and in incorporation of HDC material in RF coil constructions. Other developments would be implementing HDC-pads and other embodiments of the present invention to the MRI systems with different static magnetic field strengths available.

While the present invention has been illustrated by the description of exemplary embodiments thereof, and while the embodiments have been described in certain detail, there is no intention to restrict or in any way limit the scope of the claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to any of the specific details, representative devices and methods, and/or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept. 

What is claimed:
 1. A method for enhancing the performance of an imaging system, comprising: (a) providing an imaging system, wherein the imaging system is a magnetic resonance system; (b) providing an object, (i) wherein the object is imaged by the imaging system, (ii) wherein the object being imaged has a shape, (iii) wherein the object being imaged includes a region of interest therein, and (iv) wherein the object being imaged has a first dielectric constant; (c) providing a pad, (i) wherein the pad conforms to the shape of the object being imaged, (ii) wherein the pad contains a material having a second dielectric constant, and (iii) wherein the second dielectric constant is higher than the first dielectric constant: (d) surrounding the pad and the object being imaged with at least one radio frequency coil; (e) using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; (f) using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; (g) using the at least one coil to convert the detected magnetic field into detectable electrical signals; and (h) increasing signal to noise ratio of detected electrical signals and decreasing required power input to the at least one coil through the placement of the at least one coil around the pad.
 2. The method of claim 1, further comprising selecting the material in the pad from the group consisting of ceramics, deuterium-based gel, water-based gel, suspensions including dielectric additives, and combinations thereof.
 3. The method of claim 1, further comprising providing the pad in the form of a helmet that conforms to a portion of the object being imaged.
 4. The method of claim 1, further comprising providing the material contained within the pad in a uniform thickness.
 5. The method of claim 1, further comprising adapting the pad to receive different volumes of high dielectric constant material therein.
 6. The method of claim 1, further comprising reducing the required power input by 50%.
 7. The method of claim 1, further comprising increasing the signal to noise ratio by 27%.
 8. A method for enhancing the performance of an imaging system, comprising: (a) providing an imaging system, wherein the imaging system is a magnetic resonance system; (b) providing an object, (i) wherein the object is imaged by the imaging system, (ii) wherein the object being imaged has a shape, (iii) wherein the object being imaged includes a region of interest therein, and (iv) wherein the object being imaged has a first dielectric constant; (c) providing a pad in the form of a helmet adapted to receive different volumes of material, (i) wherein the pad conforms to the shape of the object being imaged, (ii) wherein the pad contains a material having a second dielectric constant, and (iii) wherein the second dielectric constant is higher than the first dielectric constant; (d) surrounding the pad and the object being imaged with at least one radio frequency coil; (e) using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; (f) using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; (g) using the at least one coil to convert the detected magnetic field into detectable electrical signals; and (h) increasing signal to noise ratio of detected electrical signals and decreasing required power input to the at least one coil through the placement of the at least one coil around the pad.
 9. The method of claim 8, further comprising selecting the material in the pad from the group consisting of ceramics, deuterium-based gel, water-based gel, suspensions including dielectric additives, and combinations thereof.
 10. The method of claim 8, further comprising providing the material contained within the pad in a uniform thickness.
 11. The method of claim 8, further comprising reducing the required power input by 50% and increasing the signal to noise ratio by 27%.
 12. A method for enhancing the performance of an imaging system, comprising: (a) providing an imaging system, wherein the imaging system is a magnetic resonance system; (b) providing an object, (i) wherein the object is imaged by the imaging system, (ii) wherein the object being imaged has a shape, (iii) wherein the object being imaged includes a region of interest therein, and (iii) wherein the object being imaged has a first dielectric constant; (c) providing a pad, (i) wherein the pad conforms to the shape of the object being imaged, (ii) wherein the pad contains a material having a second dielectric constant, and (iii) wherein the second dielectric constant is higher than the first dielectric constant; (d) surrounding the pad and the object being imaged with at least one radio frequency coil; (e) using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; (f) using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; (g) using the at least one coil to convert the detected magnetic field into detectable electrical signals; and (h) increasing signal to noise ratio of detected electrical signals by 27% and decreasing required power input to the at least one coil by 50% through the placement of the at least one coil around the pad.
 13. The method of claim 12, further comprising selecting the material in the pad from the group consisting of ceramics, deuterium-based gel, water-based gel, suspensions including dielectric additives, and combinations thereof.
 14. The method of claim 12, further comprising providing the pad in the form of a helmet that conforms to a portion of the object being imaged.
 15. The method of claim 12, further comprising providing the material contained within the pad in a uniform thickness.
 16. The method of claim 12, further comprising adapting the pad to receive different volumes of high dielectric constant material therein. 